Measurements are taken of the exact distances apart of the radium lines and certain adjacent iron lines used as standards. By using a formula, first communicated to the British Association by Sir George Stokes, the wave-lengths of the unknown lines can be calculated. At the time the formula was given it was sufficiently accurate for the instrumental means employed for photographing spectra, but the formula only gave approximate results, and the accuracy of determination of wave-lengths has since improved so much that a correction is required to the original formula. Sir George Stokes, before whom I placed the difficulty in June, 1895, quickly solved it in a satisfactory manner. The usual formula requires the positions of two standard lines of known wave-length, n1 and n3, one on each side of the unknown line n2. To make the small correction Sir George advised me to take a third line of known wave-length, chosen well-removed from the selected known lines n1 and n3. If chosen in the interval 1–3 it had better not be greatly distant from the middle. There is, however, very wide latitude for choice in this respect. All these lines must be photographed and measured in the usual way. Calculate the approximate wave-length of the unknown line by the original formula, and then calculate the approximate wavelength of the third known line by the same formula, as if it were unknown, using the two original standards for this purpose also.